Color filters are used in numerous applications, for example to attenuate an overly large luminous intensity or to increase an image contrast. In particular, tinted ophthalmic lenses, especially solar protection lenses, are color filters which are intended to be fitted into a spectacle frame.
Theoretically, a filter can exhibit a transmission spectrum which is substantially flat in the visible light interval 380 nm-780 nm (nanometer). However, such a filter with flat spectrum cannot be achieved in practice, especially on account of spectral limitations of the dyes which are used to manufacture filters. Moreover, the grey appearance of a flat-spectrum filter would be rather unaesthetic and rather unappealing for ophthalmic applications. The spectrum of a real filter therefore exhibits variations in the visible light interval. The filter is then liable to modify the perception of hues by an observer, between a direct observation of samples of these hues and an observation of them through the filter. For this reason, a real filter is called a color filter.
The improving of a color filter is generally a lengthy and difficult task. Specifically, not only must the hue of the filter be adjusted as a function of aesthetic or marketing criteria, but the filter must not overly much alter the hues of objects which are observed through it. Stated otherwise, a natural rendition of hues by the filter is sought. In order to propose an improvement of an initial color filter, it is then necessary to manufacture a range of color filters which correspond to as many modifications made to the initial filter, and to compare the modified filters with one another and with the initial filter as regards the restitution of hues by the filters. Such a mode of operation is lengthy and expensive, especially because a set of observers must be summoned to compare the filters. Furthermore, the filter evaluations which are produced by the observers are prone to variations, on account of their subjective nature. It is then necessary to repeat the evaluations performed by the observers, in order to subsequently calculate an average of the results obtained.
To alleviate these difficulties, document EP 1,273,894 proposes a method of obtaining a range of color filters, which is based on calculating chromatic deviations expressed in the CIELAB space, defined by the Commission Internationale de l'Eclairage. The involvement of observers is then no longer necessary for visually evaluating samples of hues seen through each filter. Nevertheless, the method which is described in this document is based on the initial manufacture of real test filters, this being lengthy and complex and in practice limits the evaluations to easily achievable filters. Furthermore, the chromatic deviations calculated give a poor account of the impression, for a human observer, of a rendition of hues which is natural.
An object of the present invention is to propose a method of improving a filter, which does not have the preceding drawbacks.